1. Field of the Invention
The present invention relates to devices and methods for selective actuation of wellbore tools. More particularly, the present invention is in the field of control devices and methods for selective firing of a gun assembly.
2. Description of the Related Art
Hydrocarbons, such as oil and gas, are produced from cased wellbores intersecting one or more hydrocarbon reservoirs in a formation. These hydrocarbons flow into the wellbore through perforations in the cased wellbore. Perforations are usually made using a perforating gun loaded with shaped charges. The gun is lowered into the wellbore on electric wireline, slickline, coiled tubing, or other conveyance device until it is adjacent the hydrocarbon producing formation. Thereafter, a surface signal actuates a firing head associated with the perforating gun, which then detonates the shaped charges. Projectiles or jets formed by the explosion of the shaped charges penetrate the casing to thereby allow formation fluids to flow through the perforations and into a production string.
In some applications, two or more guns or gun compartments are assembled to form a gun train. It is common practice to sequentially fire such perforating gun trains. Each gun is made up of a number of shaped charges, each of which is contained in a separate gun compartment. The shaped charges are usually fired sequentially, beginning at the bottom of the gun or gun compartment. The first shaped charge to be fired is connected to a ground, and the firing of that shaped charge will, unless there is a malfunction, result in the removal of that ground connection and grounding the next shaped charge in the sequence. The firing of each shaped charge, unless there is a malfunction, will result in the removal of the ground connection for that shaped charge and grounding the next shaped charge in the sequence.
Another conventional method for detonating the perforating guns includes a rotary switch operated at the surface with which the several charges can be detonated. This method, however, has its disadvantages, primarily in that the number of charges which can be detonated in this manner is limited. Another conventional method permitting sequential “select fire” detonation of the charges starting at the bottom of the gun assembly, by sequentially applying direct current (d.c.) voltage of alternating polarity to the logging cable from the surface. In accordance with this method, the logging cable is electrically connected through a diode to the blasting cap attached to the charge on the bottom of the gun assembly, and this blasting cap is grounded. All other blasting caps attached to the other charges above the bottom charge are not grounded. Instead they are electrically connected to the diode and a dart which is mounted through an insulating gasket to the baffle plate. The diode is also connected to the logging cable. The dart is a device, well known in the trade, that seals the baffle from the portion of the gun assembly below, when the charge immediately below the dart has been detonated. Other conventional selective firing devices include multiple wire-multiple shot perforating guns. In these devices, a plurality of separate circuits are employed to fire a like plurality of small groups of perforating elements. Another conventional selective firing system is the single wire-multiple shot gun. In devices of this type, there are provided a plurality of spaced normally disarmed blasting cap-perforating element assemblages and an armed assemblage. When the armed assemblage is fired, the adjacent blasting cap-perforating element assemblage is armed through the use of a mechanically operated switch.
These conventional select fire systems for various reasons, such as capacity, reliability, cost, and complexity, have proven inadequate. The present invention addresses these and other drawbacks of the prior art.